The present invention relates to a method for equalizing the temperature profile of glass sheets in a roller-equipped furnace included in a horizontal tempering plant, wherein the glass sheets are carried on a horizontal level by means of a conveyor consisting of horizontal rollers through a furnace, the opposite surfaces of a glass sheet being exposed to a conduction, convection and radiation heat effect caused by resistance elements, rollers or the like furnace components fitted above and below a glass sheet whereby, in order to equalize the total heat effect applied to the upper and lower surfaces of a glass sheet, the upper surface of a glass sheet is exposed at least in the early stage of a heating cycle to an intensified convection heat effect by blasting air into the furnace near the upper surface of a glass sheet as narrow jets which, through the injector action, create a turbulent flow of the hot air contained in the furnace along the upper surface of a glass sheet.
The invention relates also to an apparatus for equalizing the temperature profile of glass sheets in a roller-equipped furnace included in a horizontal tempering plant, said apparatus comprising a furnace, heating resistances thereinside for maintaining the furnace temperature close to a glass softening temperature, horizontal rollers inside the furnace for supporting a horizontal class sheet and providing a conveyor therefor, as well as a blasting manifold above a bearing surface consisting of the rollers for blasting heat-equalizing air into the furnace.
This type of method and apparatus are prior known from the Applicant's U.S. Pat. No. 4,390,359. In this prior known apparatus, a convection blasting manifold is used to prevent the arching of a glass sheet. When a cold piece of glass is brought into a hot, roller-equipped furnace having a temperature of more than 700.degree. C., the glass will first vigorously curve in a manner that the glass edges rise upwards. This is a natural phenomenon as the rollers deliver heat to the glass more rapidly from below than what is received by the glass from the top section of a furnace. The reason for this is a high conduction of heat from rollers to glass, which together with the convection and radiation heat below the class results in the fact that the amount of heat transferred to the lower surface of glass exceeds that transferred to the upper surface of glass through convection and radiation. This is why a convection blasting manifold is used to create forced convection on the upper surface of glass so as to compensate for the higher heat transfer to glass from below.
Thereafter, the convection blasting has been developed further in a manner that the air to be blasted is preheated in the proximity of rollers below the glass by passing a blasting-air supply pipe through a space below the glass. This method is disclosed in more detail in the Applicant's Patent application FI 894191. This prior method involves a principle of picking up the preheating heat from rollers and from air below the glass. This method was capable of increasing the power of convection blasting while decreasing the amount of air to be blasted.
Although a convection blasting method has been capable of substantially improving the quality of glass obtained from a horizontal tempering plant, the method nevertheless still involves certain deficiencies:
When oscillating glass from end to end in a furnace, the ends of loadings have been hotter than the other parts. This can be explained by the fact that convection blasting heats the rollers instead of the upper surface of glass whenever the glass does not lie upon the rollers. This is what happens at the ends of a furnace during the oscillating movement of glass. As a consequence of this, the temperatures in the ends of glasses at the ends or gables of a furnace are higher than in the rest of a load. The trailing end of a last piece of glass and the leading end of a first one can have temperatures which are up 15-20.degree. C. higher hotter than the temperature in the rest of the batch. This results in a so-called pleating effect, i.e. each end of a piece of glass in the glass advancing direction tends to bend to an arched shape, as shown in FIG. 2. Uneven or unequal heating leads also to a non-uniform grain distribution. PA1 When oscillating glass on a certain section of the length of a furnace (the glass shifting stepwise forward so that the oscillating spot travels gradually from inlet to outlet end in a furnace), there will occur situations when the furnace only contains e.g. just one minor-sized piece of glass (e.g. an automotive side window). Thus, convection blasting heats nearly all rollers contained in a furnace. This will increase the temperature of rollers only to pronounce the arching effect and this, again, requires more convection blasting. This problem has only been corrected after a lengthy loading (rollers have been cooled by loading), with a lot of incorrect glass produced by this time. The situation will be particularly problematic when starting the production of a small series without preparing for a high waste percentage. PA1 When subjecting a furnace to heavy loading, the rollers begin to cool. The rollers may even cool to such a degree that the glass receives less heat from below than from above, even if convection blasting were not used at all. A consequence of this is the curving or arching of glass downwards and, if the rollers are allowed to cool even further, the result will be a situation that glass begins to shatter in a chiller at the tempering stage. If the cooling of rollers were even or uniform (equal cooling in each roller), the problem would not be too serious but the heating of rollers could be intensified during the unloaded period of a furnace by keeping the convection blasting switched on (when oscillating from end to end in a furnace, there is always an unloaded period between batches so that the furnace would have time to reach the thermal equilibrium lost upon loading). An often encountered problem is, however, that the rollers in a certain section cool more rapidly than the other rollers. One factor contributing to this is e.g. the way a furnace is loaded. The problem zone is generally the mid-section rollers of a furnace (in the case of large-size glasses) and the rollers in the proximity of a loading gate (in the case of thin glasses and heavy loading). In these cases, it would be important to only intensify the heating of rollers within a certain section.
A common aspect to all above problems is that it must be possible to focus the convection blasting on a right spot at a right time. It is also essential that the convection effect be as effective as possible with as small an amount of air as possible since the ample use of blasting air creates further problems. First of all, the discharging air produces an extra heat load on undesired spots, leading to problems in terms of sealing and bearing systems. In addition, the air to be blasted must be heated to the temperature of the furnace air, which requires heating effect. The ample use of air colder than the furnace air may also produce problematic, hard-to-control flows in the furnace.